Whitney Heather M, Gochberg Daniel F, Gore John C
Vanderbilt University Institute of Imaging Science, Nashville, TN 37232-2675, USA.
Phys Med Biol. 2008 Dec 21;53(24):7107-24. doi: 10.1088/0031-9155/53/24/007. Epub 2008 Nov 26.
The response to radiation of polymer gel dosimeters has most often been described by measuring the nuclear magnetic resonance transverse relaxation rate as a function of dose. This approach is highly dependent upon the choice of experimental parameters, such as the echo spacing time for Carr-Purcell-Meiboom-Gill-type pulse sequences, and is difficult to optimize in imaging applications where a range of doses are applied to a single gel, as is typical for practical uses of polymer gel dosimetry. Moreover, errors in computing dose can arise when there are substantial variations in the radiofrequency (B1) field or resonant frequency, as may occur for large samples. Here we consider the advantages of using magnetization transfer imaging as an alternative approach and propose the use of a simplified quantity, the magnetization transfer proportion (MTP), to assess doses. This measure can be estimated through two simple acquisitions and is more robust in the presence of some sources of system imperfections. It also has a dependence upon experimental parameters that is independent of dose, allowing simultaneous optimization at all dose levels. The MTP is shown to be less susceptible to B1 errors than are CPMG measurements of R2. The dose response can be optimized through appropriate choices of the power and offset frequency of the pulses used in magnetization transfer imaging.
聚合物凝胶剂量计对辐射的响应通常是通过测量核磁共振横向弛豫率作为剂量的函数来描述的。这种方法高度依赖于实验参数的选择，例如 Carr-Purcell-Meiboom-Gill 型脉冲序列的回波间隔时间，并且在将一系列剂量应用于单个凝胶的成像应用中很难优化，这在聚合物凝胶剂量测定的实际应用中很典型。此外，当射频（B1）场或共振频率存在大幅变化时，如大样本中可能出现的情况，计算剂量时会出现误差。在此，我们考虑使用磁化传递成像作为替代方法的优势，并提出使用一个简化量，即磁化传递比例（MTP）来评估剂量。该测量可通过两次简单采集进行估计，并且在存在一些系统不完善源的情况下更稳健。它对实验参数的依赖性也与剂量无关，从而允许在所有剂量水平同时进行优化。结果表明，与 R2 的 CPMG 测量相比，MTP 对 B1 误差的敏感性更低。通过适当选择磁化传递成像中使用的脉冲的功率和偏移频率，可以优化剂量响应。
